Fungicide stops frogeye leaf spot

Elton Robinson Farm Press Editorial Staff | Apr 25, 2003

Frogeye leaf spot is changing the way many west Tennessee farmers think about foliar fungicides for soybeans. The disease, caused by the fungus Cercospora sojina, appears about the time the plant begins blooming, according to University of Tennessee plant pathologist Melvin Newman. It reduces yield from 5 percent to 25 percent “depending on how susceptible that variety is.”

Frogeye leaf spot is most likely to become a problem if infected seed is planted or if the disease occurred in the previous year's soybean crop and the land is not rotated. Extended periods of wet weather during the growing season will favor disease development.

Newman noted that recent studies at the University of Tennessee's Milan Experiment Station indicate that growers can get an 8- to 10-bushel increase by spraying a susceptible variety with a fungicide. Yield increases in non-susceptible varieties could be as much as 6 bushels.

Newman recommends that fungicide sprays be made at early pod set, “unless the frogeye comes in earlier than that. The key is you can't wait too long. If the leaf is even 50 percent spotted up, that's way too late. The spots keep coming for a little while even after you spray. You have to catch it early.”

Crook applied a foliar fungicide on most of his soybean acres in 2002, the first time he had done so. While Crook didn't run side-by-side trials to determine the yield benefit, “We had never had yields like that before last year. We were bumping 60 bushels an acre on a lot of our fields. I had never seen over 40 bushels before.

“You could look at the beans and see there was very little damage. They were bigger and had a shiny seed coat.”

There were other incentives for applying a fungicide, noted Crook. “On my hill ground, I grow seed for Tennessee Farmers Co-op and Hurt Seed Co. They watched my beans and advised me when it was about time to spray. Tennessee Farmers Co-op even paid a little premium to put out the fungicide program. They saw where they were getting a better quality bean by doing so.”

As for his “oil mill” beans, Crook took advantage of an incentive program with his local chemical supplier to use a fungicide. “It seemed like a no-lose situation.”

Timing of the application is important, too, according to Crook, who used the fungicide Quadris. “If you put it down too early, it seems like it plays out before harvest. We try to get the timing down where we know it will carry us all the way through.”

Good coverage is also necessary if you want to get the benefits of a foliar fungicide, added Newman. “Coverage is the name of the game. If it's not covered, it's not going to be protected.”

Crook also rotates varieties and crops as much as possible to try and keep disease in check. “But we try to stay with the latest and the best beans on the market. I also grow a fair amount of conventional beans.”

That includes Hutcheson and a new variety from Missouri, Anand, which is tolerant or resistant to many soybean diseases. Roundup Ready varieties included Pioneer and Delta King.

“I don't like to put all my eggs in one basket,” said Crook, when asked why he continued to raise conventional varieties. “I like the convenience of Roundup, but we have some fairly clean fields where we can keep the conventionals clean for about the same money, plus we're saving a good bit on seed costs.”

Newman noted that the UT Extension Service has ratings on about 200 soybean varieties for frogeye, stem canker, sudden death syndrome, and a number of other diseases. Newman urges growers “to use our data to find out whether or not they have a susceptible variety to frogeye. If they do, that should provide additional incentive to be ready to spray.”

But keep an eye on yield potential, noted Newman. “If things turn off bone dry and you're obviously not going to make 25 to 30 bushels, it may not be feasible to treat foliarly, but if yield potential looks good and we have plenty of moisture for fungal growth, and you have a susceptible variety, I'd think seriously about spraying it.”

Newman added that Quadris was one of the top-performing fungicides in his soybean test plots. The product “increased germination, decreased defoliation and significantly decreased the diseases.”

“At the 12-ounce rate, we got an 8-bushel increase at the Milan Experiment Station and at the 6-ounce rate, we got a 6-bushel increase.”

SDS is a growing concern, “especially in corn-growing regions of west Tennessee where corn is rotated with soybeans,” according to Newman. “It shows up around pod-fill and can cut the yield 25 percent to 50 percent. Some stem canker is still around on susceptible varieties.”

Newman reports that a new race of soybean cyst nematode, race 2, is becoming more prevalent in west Tennessee. “Half to 60 percent of those infested soybean acres is now race 2.”

Foliar fungicides are less effective for soybean producers in the Missouri Bootheel, according to Allen Wrather, plant pathologist at the University of Missouri Delta Center in Portageville, Mo.

Foliar-applied fungicides conducted in tests at the Delta Center in Portageville and on farmer fields “rarely results in a yield increase,” Wrather said. “And if it is, it's small and not sufficient to pay for their investment.

“The difficulty is predicting if the disease will advance enough to cut yield,” Wrather said. “And that's dependent on the weather. There have been cases when frogeye leaf spot has developed, gotten onto the pods and yields have been reduced and fungicides would have been well worth the money. But it's really rare and it's almost impossible to predict.”